This invention relates to a piezoelectric polymeric material in the form of sheet or film, which is a polymer mixture comprising a vinylidene fluoride base resin.
Piezoelectric materials are of wide application in various fields of technology and industry, and they have been used in various pressure-sensitive devices, ultrasonic transducers and many other kinds of transducers for the audio, medical and scientific research purposes.
Piezoelectricity is an energy-converting function known as specific to crystals that have no center of symmetry, and the piezoelectric materials now in practical use are mostly inorganic materials such as quartz and PZT (lead titan-zirconate ceramics) and very limited organic crystalline materials such as Rochelle salt. These materials are generally high in the piezoelectric moduli, but they are hard and brittle materials that cannot easily be processed into various shapes. Therefore, especially it is especially difficult to produce a thin sheet or film of a conventional piezoelectric material with large widths.
Meanwhile, it has been discovered that some kinds of polymeric materials including some natural polymers such as collagen and cellulose and some synthetic polymers typified by poly(vinylidene fluoride) and poly(.gamma.-methyl-L-glutamate) exhibit piezoelectricity when films of such polymers are subjected to a DC electric field treatment in a stretched and heated state and then cooled while the electric field is maintained. Presently it is common knowledge that, among the hitherto investigated synthetic polymers, poly(vinylidene fluoride) in the form of a stretched film exhibits the highest modulus of piezoelectricity. However, even the piezoelectric modulus (d.sub.31) of poly(vinylidene fluoride) in the form of a stretched film is about 5.times.10.sup.-7 cgs-esu at the most. Therefore, great restrictions are placed on the practical use of poly(vinylidene fluoride) as a piezoelectric material.